1. Field of the Invention
This invention relates in general to microwave phase-shifting apparatus and in particular to phase shifters with either digital or analog control mechanisms for use with microwave signals being transmitted over transverse electromagnetic lines.
2. Description of Related Art
A phase shifter may be generally defined as a two-port microwave device which is used to control the phase of a signal passing therethrough. Many types of phase shifters are known including diode phase shifters of the constant time delay type and the constant phase shift type, and ferrite phase shifters which are generally made by placing ferrite materials in a section of wave guide. Ferrite phase shifters can be either of a reciprocal or non-reciprocal type, depending upon the applied DC magnetic bias and the configuration. In such devices the phase-shifting is accomplished by changing the magnetic permeability of the ferrite material by altering the external applied DC magnetic field. A solenoid made by winding several hundred or thousand turns of wire about a rectangular waveguide section often is used to supply a longitudinal magnetic field in such devices. One such ferrite phase-shifting device is described in F. Reggia & E. Spencer, "A New Technique in Ferrite Phase Shifting For Beam Scanning of Microwave Antennas", Proceedings of the IRE, November, 1957, pp. 1510-1517. The characteristics of such ferrite shifters are now well-known in the art, and they are often used in many phased array antenna systems for beam scanning.
The ferrite phase shifters in comparison with a diode phase shifter has two primary advantages, namely higher power handling capability, and lower insertion loss because of the waveguide configuration. However, such ferrimagnetic microwave phase shifters also have the following disadvantages: (1) a high DC power consumption; (2) a relatively high temperature sensitivity; (3) relatively heavy-weight per unit power handling capability; and (4) relatively high insertion loss, especially if the input signal is increased beyond a certain power level.
Another type of phase shifter sometimes used in microwave circuits, at least in laboratory set-ups, is a piece of dielectric film placed in a portion of a hollow waveguide. These phase shifters operate on the principle that the propagation velocity of microwave energy passing through a medium is decreased, in comparison to its velocity through air, by the ratio 1/.epsilon..sup.1/2, where .epsilon. is the dielectric constant of the mdeium relative to the air. The extra transit time required by a microwave signal of a given frequency to pass through a predetermined length of the medium in comparison to time required to pass through an equivalent length of hollow, air-filled waveguide, provides a negative phase shift whose value is proportional to the given frequency.
Recently, a new coaxial phase shifter for transverse electromagnetic line was developed at Hughes Aircraft Company, the assignee of the present invention. This phase shifter, constructed in line with the inner conductor of the transmission line by using an interdigitated finger design, is very compact and does not require alteration of the transmission line's outer conductor to implement. This phase shifter is fully described in U.S. Pat. No. 4,616,195, granted Oct. 7, 1986 to R. Ward et al. The disclosed phase shifter is preferably used for a "squarax" transvere electromagnetic (TEM) transmission line that is a coaxial transmission line whose inner and outer conductors are square in cross-section. The phase shifter has the advantage of being a very compact and non-complex structure with a low voltage standing wave ratio (VSWR). The patent discloses that the amount of phase-shifting of the signal being transmitted selectively varied by altering the spacing between the in-line fingers extending toward one another from the left and right segments of the inner conductor. It also discloses that phase shifts larger than those provided by one phase shifter of the kind just described may be achieved by cascading two or more separate phase shifters in series with each other. However, this patent does not disclose any technique for providing, with this phase shifter, a mechanically or electrically adjustable phase shifter to a microwave signal being transmitted therethrough.
In many microwave applications, such as phased array antenna systems used in ground or space-based locations, it is necessary to provide a phase shifter which is adjustable. The ferrite phase shifters previously described may be automatically adjusted, but they still suffer from the aforementioned disadvantage.
Another type of adjustable phase shifter often used to adjust the electrical separation of microwave components without introducing additional impedance mismatch are line stretchers. These devices include one or more hollow telescoping pairs of metallic tube whose overall length can be physically changed by sliding the inner tube further into or out of the outer tube. However, they generally are physically quite large in size and, as far as we are aware are simply manually adjusted to the desired length and then locked down or tightened up to prevent further movement.
It would be most advantageous to have a compact, relatively lightweight, adjustable phase-shifting apparatus that can be automatically operated for various microwave circuit applications, such as phased array antenna systems, and particularly for space applications where low weight, low power consumption and simplicity are highly valued.
The object of the present invention is to overcome the foregoing disadvantages of ferrite phase shifters, and to provide an adjustable phase-shifting apparatus which is relatively light weight, inexpensive and simple to operate. Other objects of the present invention includes: providing a phase-shifting apparatus suitable for use with squarax transmission line, providing a phase-shifting apparatus which may be automatically controlled digitally or in an analog fashion; providing a phase-shifting apparatus which may selectively provide multiple phase shifts of different values in response to external commands; and providing an interconnected pair of phase shifters operated by a mechanical linkage which produce equal but opposite phase shifts in a pair of microwave signals, for applications such as a phased array antenna.